This invention relates to material treatment systems that employ a gaseous medium to fluidize particles in heat exchange or other treating relation and more particularly to particulate treatment systems suitable for use with transport mechanisms of the belt conveyor or similar type.
Particulate material is advantageously treated by maintaining the particles in fluidized conditions as they are transported through a particle treatment zone. The particles may be fluidized by a gas flow that is in heat exchange or other treating relation with the particles. Such systems find extensive use in the food industry for processing particles such as coffee beans, grains, cereal flakes, fruit, etc., and in other industries for promoting or retarding chemical reactions, for driving off free or absorbed liquids or moisture or for otherwise conditioning granular, pulverent and other particulate materials.
In a conventional air fluidizing treatment system, variations in the treatment time of the product particles may cause non-uniformity in the finished product. With certain food products having relatively short treatment times at high temperatures (e.g., puffed snack foods), this variation may be detrimental. For example, puffed corn curls are often in pellet form before being treated. Normally, no more than 30 to 40 seconds of exposure to heated air is required for the pellets to expand as much as ten times in size. If the pellets are under-treated, they may only expand partially or not at all. On the other hand, over-treating the curl product will result in burning and discoloration. The expansion of the curl product exacerbates the problem of forward and backward excursion since their light weight and increased surface area in their puffed state allows them to be randomly and more easily thrown about the conveyor bed.
Another example of a product which might be desirably treated in such a system is infused blueberries. Infused blueberries are permeated with a sugar-sweetened syrup and then treated within the system to provide partially dried blueberries which are typically blended, for example, with ready-to-eat breakfast cereals or baked goods. Unless they are continually tumbled and separated from each other, the infused blueberries may agglomerate into clusters making them difficult to dry. The velocity of the heated air must be high enough to overcome particle to particle adhesion, but vigorous fluidization can cause product carryover, loss in the exhaust air stream, excessive product contact and deposits on the air delivery tubes and treatment chamber walls. Moreover, vigorous fluidization can cause the loss of residence time distribution control which adversely affects the uniform treatment of the product.